ARRAS France (Reuters) - The Tour de France has not even reached the mountains and Vincenzo Nibali has already built a sizeable advantage over his main rivals, yet hard work still lies ahead if the Italian is going to complete his collection of grand tour victories. After a mediocre start to the season, Nibali has hit top form on the Tour, winning the second stage in aggressive style before strengthening his advantage with an impressive display on the cobbles in Wednesday's fifth stage.